I have a 06 f-450 supper cab utility truck with a 6.0 diesel i use for heavy long distant towing.The truck has 48,000 miles on it can i change the gear oil to synthetic and if so will it help on fuel mileage.Thanks

 

Don't know if it will help the mileage but synthetic will be fine. If you have Limited slip you need to get it with limited slip additive. 75W-140

 

I believe it may have synthetic oil in from factory. The rear axle is a Dana axle (should be a D80) so I believe it takes a 75w90 or 80w90 gear oil.(check your owners manual) But I would put synthetic in it regardless. It is easy to change diff oil just use the proper sealant to seal the cover.